Inflammation Is a complex process involving the interplay between leukocytes that infiltrate the tissue, cells that are resident in that tissue, and factors produced by both populations of cells. An additional layer of regulation and complexity are the changes in the composition ofthe extra cellular matrix that is associated with sites of inflammation. The nature ofthe factors that control these interactions is not at all clear. There is considerable evidence that the cytokine thymic stromal lymphopoietin (TSLP) is critical for the development and progression of Th2-type allergic inflammation in humans and rodents. While the importance of TSLP to allergic inflammation is becoming clear, its role in other inflammatory responses, especially to pathogens, is less clear. This is especially important for pulmonary infection with respiratory syncytial virus (RSV), which can induce both protective Thi responses and non-productive Th2 responses in humans and susceptible mouse strains. It has been postulated that the Th2 response induced by RSV is also responsible forthe RSV-induced exacerbations seen in asthmatics. Several studies have shown both epidemiologically in humans and in mouse models that there is a clear association between severe RSV infection and long-term functional impairment of the lungs, including the development of asthma. The data described above suggest that both TSLP and RSV are capable of driving potent Th2-type responses in the lung. In addition, as will be shown below, both elevated TSLP and RSV infection result in a marked increase in the deposition ofthe proteoglycan versican in the lung. Taken together, these data strongly suggests a functional link between RSV infection and TSLP in driving pathological responses in the lung. Consistent with this model, RSV infection of airway epithelial cells induces robust TSLP gene expression. The experiments in this proposal are designed to test the hypothesis that it is RSV-induced TSLP that is responsible for the Th2 response to infection and for the virus-induced asthmatic exacerbations, and that it is the interplay between infiltrating cells and versican that drives this inflammatory response.